The first direct experimental confirmation in a laboratory that light carries momentum was accomplished by E. F. Nichols and G. F. Hull in the U.S. (Nichols E. F., Hull G. F., Phys. Rev. 13, p.307 (1901)) and P. N. Lebedev in Russia (Lebedev P. N., Ann. Phys. (Leipzig) 6, p.433 (1901)) around the turn of the century. In both experiments the radiation pressure of a light source was detected by the twisting motion of mirrors suspended by thin wires in a high vacuum. The vacuum was crucial to eliminate the effects of thermal or radiometric forces. No one at the time imagined that there would be any practical application for this minute effect. The first process in which photon momentum played an important role was the Compton effect, i.e. the scattering of X-rays against electrons. Another four decades passed until the invention of the laser in 1960. The possibility of producing spatially coherent light with very high intensity brought the effect of radiation pressure into the macroscopic world. An early pioneer in this field was A. Ashkin at Bell Laboratories. Ashkin was the first to use a laser for manipulating transparent, micron sized latex spheres in the late 60s. These spheres, suspended in a water solution, were first accelerated with one horizontal laser beam and then trapped in between two beams in a second experiment (Ashkin A., "Acceleration and Trapping of Particles by Radiation Pressure" Phys. Rev. Lett. 24(4) p.156-159 (1970)).
Since then, this characteristic of light has been exploited in many ways. One broad field is the trapping and cooling of atoms and molecules. The fact that this year's Nobel prize was awarded to Steven Chu, William D. Phillips, and Claude Cohen-Tannoudji for the development of methods to cool and trap atoms with laser light shows its absolute relevance to today's science (Chu S., "Laser Trapping of Neutral Particles" Sci. Am., p.71 (February 1992); Phillips W. D., Metcalf H. J., "Cooling and Trapping Atoms" Sci. Am., p.36 (March 1987); Cohen-Tannoudji C., Phillips W. D., "New Mechanisms for Laser Cooling" Physics Today, p.33 (October 1990); Chu S., "Laser Manipulation of Atoms and Particles" Science 253, p.861-866 (1991)).
The interactions of light with dielectric matter can be divided into two predominant forcing mechanisms. The gradient force, which pulls material with a higher relative index towards the areas of highest intensity in a laser beam, and the scattering force, which is a result of the momentum transfer of photons to the material. Most optical micro-manipulation tools rely on gradient forces. However, the "Optical Stretcher" of this invention uses the scattering forces to deform elastic dielectric samples.
The characteristics of these two forces (gradient and scattering force) allow for different possible trap designs.
One of the first stable traps was the levitation of particles with a vertical laser beam (Ashkin A., Dziedzic J. M., "Optical Levitation by Radiation Pressure" Appl. Phys. Lett. 19(6), p.283-285 (1971); Ashkin A., "The Pressure of Laser Light" Sci. Am. 226, p.63-71 (1972); Ashkin A., Dziedzic J. M., "Optical Levitation in High Vacuum" App. Phys. Lett. 28(6), p.333-335 (1976); Ashkin A., Dziedzic J. M., "Observation of Light Scattering from Nonspherical Particles Using Optical Levitation" Appl. Opt. 19(5), p.660-668 (1980)). The scattering force is strong enough to balance gravity while the gradient force keeps the particle on the optical axis.
A big improvement in this trap was the use of a highly focused laser beam, first realized by Ashkin in 1986 (Ashkin A., Dziedizic J. M., Bjorkholm J. E. , Chu S., "Observation of a Single-Beam Force Optical Trap for Dielectric Particles" Opt. Lett. 11(5), p.288-290 (1986)), because it is independent of gravity and one can orient the trap in any direction in space or even use it in micro-gravity (Gussgard R., Lindmo T., Brevik I., "Calculation of the Trapping Force in a Strongly Focused Laser Beam" J. Opt. Soc. Am. B 9(10), p.1922-1930 (1992)). The idea is that extreme focusing leads to a point in space (rather than an axis) with a very high intensity. Thus, the gradient force pulls a dielectric particle towards this point. The focusing has to be strong enough that the gradient force overcomes the scattering force, which is an unwanted effect in this case, because it pushes the particle away from the focus. This is usually realized by directing a laser beam through a microscope objective with high numerical aperture (NA). An experimental improvement can be achieved by using an objective with a central field stop producing a conical dark field. This enhances the relative contribution from high NA illumination and diminishes the influence of the scattering force. At the same time the particle is trapped near the focus and can be observed with the microscope. This one-beam setup, usually referred to as the Optical Tweezer, has been studied extensively (Ashkin A., Dziedizic J. M., Bjorkholm J. E. , Chu S., "Observation of a Single-Beam Force Optical Trap for Dielectric Particles" Opt. Lett. 11(5), p.288-290 (1986); Ashkin A., "Forces of a Single-Beam Gradient Laser trap on a Dielectric Sphere in the Ray Optics Regime" Biophys. J. 61, p.569-582 (1992); Wright W. H., Sonek G. J., Berns M. W., "Radiation Trapping Forces on Microspheres with Optical Tweezers" App. Phys. Lett. 63, p.715-717 (1993); Gussgard R., Lindmo T., Brevik I., "Calculation of the Trapping Force in a Strongly Focused Laser Beam" J. Opt. Soc. Am. B 9(10), p.1922-1930 (1992); Visscher K., Brakenhoff G. J., "Theoretical Study of Optically Induced Forces on Spherical Particles in a Single Beam Trap I: Rayleigh Scatterers" Optik 89(4), p.174-180 (1992); Kuo S. C., Sheetz M. P., "Optical Tweezers in Cell Biology" Trends in Cell Biology 2, p.116-118 (1992)) and is widely used in biological applications. The power of the laser used is in the range of a few mW up to 1.5 W for the trapping of glass or latex beads and the achieved trapping forces vary from Pico- to Nanonewton depending on the size and index of refraction. For detailed reviews see Kuo S. C., Sheetz M. P., "Optical Tweezers in Cell Biology" Trends in Cell Biology 2, p.116-118 (1992); Bems M. W., Wright W. H., Steubing R. W., "Laser Microbeam as a Tool in Cell Biology" Int. Rev. Cytol. 129, p.1-44 (1991); Block S. M., Optical Tweezers: A new Tool for Biophysics, in Noninvasive Techniques in Cell Biology, G. S. Foskett J. K., Editor. 1990, Wiley-Liss.: New York. p. 375-402; Greulich K. O., Weber G., "The Laser Microscope on its Way from an Analytical to a Preparative Tool" J. Microsc. 167, p.127-151 (1991); Simmens R. M., Finer J. T., "Glasperlenspiel II: Optical Tweezers" Curr. Biol. 3, p.309-311 (1993); and Weber G., Greulich K. O., "Manipulation of Cells, Organelles, and Genome by Laser Microbeams and Optical Traps" Int. Rev. Cytol. 133, p.1-41 (1992).
Although the Optical Tweezer is a very powerful tool, it also has its limitations. The working distance of high NA objectives is very short and does not allow for additional test equipment between objective and object. The trapping zone is rather small (on the order of the light wavelength). It has been shown that a good trapping efficiency can only be achieved for indices of refraction smaller than .apprxeq.1.7 because of the loss of axial stability (the scattering force becomes stronger than the backward gradient force) (Svoboda K., Block S. M., "Biological Applications of Optical Forces" Annu. Rev. Biophys. Struct. 23, p.147-285 (1994)). Furthermore, focusing the beam down to the theoretical limit of spot sizes (half the wavelength of the used light) leads to very high intensities that can endanger the integrity of biological objects. Most cells trapped with an optical tweezer do not survive powers greater than 20-250 mW because the extreme focusing leads to very high local intensities. This depends also on the specific cell type and the used wavelength, of course (Ashkin A., Dziedzic J. M., Yamane T., "Optical Trapping and Manipulation of Single Cells Using Infrared Laser Beams" Nature 330(24), p.769-771 (1987); Ashkin A., Dziedzic J. M., "Optical Trapping and Manipulation of Viruses and Bacteria" Science 235, p.1517-1520 (1987); Kuo S. C., Sheetz M. P., "Optical Tweezers in Cell Biology" Trends in Cell Biology 2, p.116-118 (1992)).
A different setup, that circumvents most of these problems, is a two beam trap (Ashkin A., "Acceleration and Trapping of Particles by Radiation Pressure" Phys. Rev. Lett. 24(4) p.156-159 (1970); Roosen G., Imbert C., "Optical Levitation by Means of Two Horizontal Laser Beams. A Theoretical and Experimental Study" Phys. Lett. 59A(1), p.6-9 (1976); Roosen G., "La Levitation Optique de Spheres" Can. J. Phys. 57, p.1260-1279 (1979); Ashkin A., Dziedzic J. M., "Optical Levitation by Radiation Pressure" Appl. Phys. Lett. 19(6), p.283-285 (1971)). Historically the two beam trap was developed more than a decade before optical tweezers but has since fallen into disuse. Two identical, counterpropagating laser beams with Gaussian beam profiles can stabilize a particle in the point of symmetry. The forces on a sphere are the superposition of the forces of two individual beams. The gradient force confines the particle to the axis while the scattering force provides stability along the axis. This is a stable configuration as long as the refractive index n of the particle is higher than that of the surrounding medium. Air bubbles in water, an example where this condition is not fulfilled, are pushed out of the beam (like a rubber ball out of a water beam). Another condition, which is somewhat unexpected, is that the diameter of the particle has to be smaller than the beam radii in the center. For the situation where the beam radius is smaller than the particle radius, and thus the ratio is larger than 1, the force closer to the waist of the beam is smaller than further away due to the divergence of the beam. This leads to an amplification of small displacements of the sphere from the center which means that the particle is not stably trapped (Roosen G., "A Theoretical and Experimental Study of the Stable Equilibrium Positions of Spheres Levitated by Two Horizontal Laser Beams" Opt. Comm. 21(1), p.189-195 (1977)).
An improvement of this setup has been demonstrated in which single mode (SM) optical fibers are used to deliver the laser beams to the trapping area (Constable A., Kim J., Mervis J., Zarinetchi F., Prentiss M., "Demonstration of a Fiber-Optical Light-Force Trap" Opt. Lett. 18(21), p.1867-1869 (1993)). This avoids the need for additional optical elements and their alignment, and allows a very easy and cheap implementation of the trap into custom made experiments. This trap can also be set up independently from a microscope. A good review of all these traps and their applications in biology can be found in Svoboda K., Block S. M., "Biological Applications of Optical Forces" Annu. Rev. Biophys. Struct. 23, p.147-285 (1994).
A new member in the family of light traps is the Optical Spanner (Padget M., Allen L., "Optical Tweezers and Spanners" Physics World, p.35-38 (September 1997)). The basic setup is as for the Optical Tweezer but instead of a Hermite-Gaussian laser profile, a Laguerre-Gaussian profile is used. These beams have a circular cross-section, a helical wavefront, and a Poynting vector that spirals around the axis. This means that such a beam has an orbital momentum in addition to the translational momentum previously discussed. Since the angular momentum of the system has to be conserved too, the particle starts to rotate. In this way particles can not only be translated but also rotated.
Although these traps are extremely useful for all kinds of manipulation of objects, they can only translocate and/or rotate them and are not intended to deform them. The Optical Stretcher of this invention expands the line of optical tools and allows for a fill spectrum of particle manipulation.
All existing methods to examine the elasticity of cells have their limitations. In micropipette aspiration experiments the tip of a micropipette is placed onto a cell with a micro-manipulator and part of the cell is pulled into the pipette with an applied negative pressure. This provides only very local information and can detach the membrane from the cell which leads to inaccurate measurements.
Another possibility is the use of an Atomic Force Microscope (AFM) in tapping mode (Radmacher M., Fritz M., Kacher C. M., Cleveland J. P., Hansma P. K., "Measuring the Viscoelastic Properties of Human Platelets with the Atomic Force Microscope" Biophys. J. 70, p.556-567 (1996)). The oscillating AFM tip is scanned across the cell body allowing the force and the indentation to be measured. Usually, the Young modulus of the cell is then determined using the Hertz model which assumes a semi-infinite slab of material and connects deformations to its material constants. The problem here is that the spring constant of the AFM tip/cantilever is rather big compared to the strength of the cytoskeleton which means that it is not possible to detect small elasticity differences of cells--the cell is either compressed or not. This treatment is also very rough, as many cells do not survive it. AFM also looks at the elasticity only over small areas of a cell's surface. Similar to this are "cell poking" experiments where the AFM tip is replaced by a glass needle (Elson E. L., "Cellular Mechanics as an Indicator of Cytoskeletal Structure and Function" Annu. Rev. Biophys. Chem. 17, p.397-430 (1988)).
Another major disadvantage, which all these methods have in common, is that they are not efficient. It is very inconvenient to position the micropipette or the AFM tip manually on a cell. Thus, it is effectively not possible to measure a significant number of cells in a short period of time which results in lack of good statistics.
A more indirect approach was to shear a whole pallet of densely packed cells with a rheometer (Eichinger L., Koppel B., Noegel A. A., Schleicher M., Schliwa M., Weijer K., Wittke W., Janmey P. A., "Mechanical Perturbation Elicits a Phenotypic Difference Between Dictyostelium Wild-type Cells and Cytoskeletal Mutants" Biophys. J. 70, p.1054-1060 (1996)). However, this is a bulk measurement and yields only mean values and not specific information about a single cell. Another limitation is that not only the cell elasticity but also sticking forces and friction between the cells influence the outcome of the measurement.